[RFC PATCH 0/6] SYSVIPC/semaphores - allow saving/restoring a process' semundo_list

[Nadia.Derbey at bull.net]

This patchset is a part of an effort to make sysv ipc objects
read/writable from userspace for checkpoint / restart.

System V ipc's are objects that are global to a system and can thus be
checkpointed and restarted on a container basis. But some parts of the ipc
structures are process related and, as such should be checkpointed / restarted
on a process basis.

Message queues needn't and thus cannot be reached from a task structure
(only the other direction is possible: a task can be reached by a msg receiver
or sender structure if it is sleeping).

Shared memories are accessible from a task structure through that task's
memory mapping (/proc/<pid>/maps shows a process' memory maps).

Semaphores are kind of accessible from a task structure too: the task
structure's sysvsem field makes it possible to walk through all the semaphores
operations to undo when a process is exiting.
This list, that need to be saved and restored during a process' c/r,
cannot yet be accessed from user space.

This is a feature that will be needed if ever we take the direction of driving
checkpoint / restart from user space, though the read part of it could be used
even from now on.

Since this undo_list is, again, on a thread basis, we propose to externalize
it via a new proc file: /proc/<pid>/semundo.

Actually, Pierre Pieffer has already done the proposal in threads
https://lists.linux-foundation.org/pipermail/containers/2008-January/thread.html#9756
up to #9759

I've ported them to 2.6.26-rc5-mm3, and I'm now coming back with a
simpler implementation: the write operation is now only allowed into
/proc/self/semundo, which simplifies the locking strategy.


These patches should be applied to 2.6.26-rc5-mm3, in the following order:
[ PATCH 1/6] : ipc_rcu_protect_access_to_undo_list.patch
               Makes the process' undo_list rcu protected in order to enable
               safely reading it.
[ PATCH 2/6] : ipc_procfs_semundo_file.patch
               Introduces the semundo proc file (the seq operations are still
               empty).
[ PATCH 3/6] : ipc_procfs_semundo_start_stop_seqops.patch
               Introduces the .start and .stop seq operations.
[ PATCH 4/6] : ipc_procfs_semundo_next_seqop.patch
               Introduces the .next seq operation.
[ PATCH 5/6] : ipc_procfs_semundo_show_seqop.patch
               Introduces the .show seq operation.
[ PATCH 6/6] : ipc_procfs_semundo_write.patch
               The semundo proc file becomes writable.

Comments are welcome!

Regards,
Nadia

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